1. Field of the Invention
The present invention relates generally to the field of metal electroplating and more particularly to a method and apparatus for nickel-chrome plating of parts with internal recesses.
2. Description of the Prior Art
Steel parts may be plated to prevent corrosion and improve appearance. Commercial plating methods many times mount small parts on racks which act as electric cathodes that are passed through numerous electro-chemical plating steps. The parts are generally attached to the rack in a fixed position. This is accomplished by providing attachment points or fingers on the rack that engage the part. These attachment points have conductive tips that act as electrical contacts with the part and also act as mechanical springs to hold the part on the rack. The part can be mounted by pushing it onto one or more of such fingers that hold the part firmly while making good electrical contact into the metal of the part. Each rack may be designed and constructed specially to hold a part of specific size and shape.
The loaded racks are then normally suspended from a rail on an automatic plating machine. This machine can have numerous cleaning, plating and rinsing stations. In the case of nickel-chrome plating, the machine usually has several cleaning stations, several nickel plating stations, a chrome plating station and several rinse stations. The parts may require several layers of nickel including a layer of anti-corrosion nickel and a layer of bright nickel as well as a layer of chromium. The loaded rack is generally moved down the rail above each station or tank. As each new station is encountered, the machine halts and lowers the rack into a tank containing an appropriate solution for that station. Stations where actual plating is performed have metal anodes of nickel or chromium in the tanks with the proper electrolyte for that plating step. As a loaded rack of parts is lowered into a plating tank plating begins since there is a voltage is applied between the rack (cathode) and the metal anode to effect plating through the electrolyte solution as is known in the art. The various solutions in the process can be agitated with a continuous flow of air or by mechanical stirring or by other methods. A typical setup has one or more cleaning tanks, four nickel plating tanks, chrome plating tanks and several rinse tanks.
There are some parts that contain recessed cavities such as the type of lug nut that has internal threads. It is very desirable to be able to plate a thin layer on the inside of the part to prevent corrosion of the threads. Usually a plating thickness of around 1 micron on the threads can be sufficient. However, if a lug nut of this type is simply placed on a rack using a standard spring finger, it has been found that no plating takes place in the threaded cavity. It is believed that this is because the cavity forms a stagnant area in the electrolyte fluid which quickly depletes of metal ions causing the plating process to stop in the cavity. It would be advantageous to have a method and system for plating parts such as lug nuts with a recessed thread cavity. Various attempts have been made to solve this problem including air venting, turning the parts upside down, and tube venting. None of these methods have been found to work satisfactorily.
Also, it has been found that even parts without recesses will not always plate at points where the holding fingers make contact. It would be advantageous to be able to plate parts with deep recesses and to prevent non-plated regions on parts where fingers or other electrodes attach.